Geochemical Characteristics of Arsenic in Groundwater of Lanyang Plain and Human Health Risk Assessment
Date Issued
2008
Date
2008
Author(s)
Lee, Jin-Jing
Abstract
Elevated arsenic (As) in groundwater has been found in the Lanyang plain of northeastern Taiwan. The 2004 monitoring data of groundwater quality by the WRA (22 drilling stations with 40 wells), EPA (15 wells), and EPB (16 wells) were adopted to explore hydrogeochemical characteristics, mechanisms of As release to groundwater, As speciation and redox zonation of aquifer in the Lanyang Plain. Boundaries of groundwater of various depths were affected by water salinization in fishponds and As pollution was first delineated by factor analysis and cluster analysis. Large extraction of groundwater by aquaculture was a major cause of salinization, which gradually expanded from uppermost aquifer to lower aquifer. The pollutants (TOC and NH4+-N) from human activities infiltrated to shallow groundwater, triggering reducing reactions, and release As. The presence of As in deep aquifers results from the degradations of organic matter in sediment, causing groundwater progressively be more reductive. Furthermore, redox zonation and As speciation in aquifers were evaluated by using discriminant analysis and geochemical modelling (PHREEQC). The reducing conditions tend to increase from the mountainous area to the coastal area in aquifers of the Lanyang plain. Additionally, analytical results of a total of 149 geological core samples from 5 drilling wells located at mid- and distal-fan area, indicating a positive correlation of As and Fe contents in marine sequences. Surface analysis of core sample surface were performed by x-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM), As was adsorbed onto/copricipitated with noncrystalline Fe hydroxides. The reductive dissolution of As-rich iron oxyhydroxide was postulated to be the major release mechanism of As into groundwater. Additionally, a total of 929 residential well water quality in the Lanyang Plain surveyed from 1997 to 1999, were adopted to evaluate the potential risk to human health and population mortality. The As concentrations estimated by indicator kriging (IK) in groundwater were high ( > 50 μg/L) at six townships−Jiao-Si, Yi-Lan, Juang-Wei, Wu-Jie, Don-Shan and Lou-Don. The estimated TR values exceeded ten times of the safe value (10-6) in the As-affected townships. The highest TR values were 2,400 times higher than the safe value. Most annual mortalities due to As-induced internal cancers occurred in the Yi-Lan township (10 cases), Lou-Don (5 cases), Wu-Jei (3 cases), Jhung-Wei (2 cases) and Don-Shan (1 case), and the highest number of mortalities per year in the study area is 24 (five fatalities per 100,000 persons). Moreover, a parsimonious model was developed to predict occurrence probability of As concentration ≧10 μg/L in groundwater by logistic regression. The model parameters were based on pH, and NH4+-N and Fe concentrations which are correctively associated with As occurrence. The percentage accuracy of total correct classification for the model was 89.8%. The model can be applied to inform local resident on potential exposure risk of As-affected groundwater. Finally, a probability map of groundwater resources for multi-purpose uses (irrigation, aquaculture and drinking water) was assessed using multiple variable indicator kriging (MVIK). The model was based on strictest criteria of multi-purpose uses, and hydrochemical parameters were classified four main hazard categories – saline hazard, nitrogen hazard, As hazard and Fe-Mn hazard. Analytical results demonstrate that deep aquifer has a high hazard rating and is less safe than the shallow aquifer. The Fe-Mn hazard in Lanyang Plain groundwater is present in most aquifers, and is partially combined with other hazards, such as the nitrogen hazard and the As hazard, thereby forming other hazards. Safe and potentially hazardous groundwater regions for multi-purpose uses were delineated according to estimated probabilities of 0.25, 0.5 and 0.75. Thus, a zonal management plan based on safe groundwater use is formulated. This plan is useful to local governments in developing groundwater resources in the Lanyang Plain.
Subjects
Arsenic (As)
geochemical modelling
health risk
indicator kriging
logistic regression
scanning electron microscopy (SEM)
X-ray photoelectron spectroscopy (XPS)
multi-variable indicator kriging (MVIK)
Type
thesis
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